Visual electrophysiology measurement

This chapter discusses the anatomy and physiology of vision. It describes some of the more commonly used electrophysiology tests. PRVEP has low variability and provides strong clues to overall visual function. As it is a cortical response, it is often thought to be a cortical test or an optic nerve test. In fact a pristine PRVEP requires good fixation, concentration, optics capable of resolving the stimulus, well functioning retina, optic nerve and cortex. If any of these are altered the PRVEP can be reduced. Of note the PRVEP only measures up to the primary visual cortex, if a lesion is beyond this then it is possible to record a good PRVEP but still have cortical visual loss. FVEP is a much more robust and rudimentary test of vision. It has great use when opacities are present or cooperation is limited (paediatrics). Due to variation it may have limited input except when making inter-ocular or inter-hemispherical comparisons. Multi-channel VEP is useful in detecting lesions at the chasmal level or beyond. PERG is a very useful adjunct to the PRVEP as it provides information about how the sensor (eye) is performing to the same sort of stimulus. Unless a PERG is performed it is difficult to be certain whether an abnormal PRVEP is due to optic nerve damage (or beyond) or whether it is due to macular dysfunction or poor optics. mfERG provides topographic information about function of the macular. mfERG is thought to be more robust to changes in optics than PERG, and can detect localized areas of dysfunction. ERG provides clues to the health of the rod and cone pathways; and whether pathology is at the photoreceptor layer, bipolar cell layer or beyond. EOG is a measure of the RPE-photoreceptor complex and is useful in diagnosing Bests Macular Dystrophy where the ERG is normal and the EOG is badly affected. It will be reduced in disease with pan retinal dysfunction (often in these cases the a-wave is very reduced).